Multiple-Input Multiple-Output (MIMO) technology will hereinafter be described in detail. In brief, MIMO is an abbreviation for Multiple Input Multiple Output. MIMO technology uses multiple transmit (Tx) antennas and multiple receive (Rx) antennas to improve the efficiency of transmit/receive (Tx/Rx) of data, whereas the conventional art generally uses a single transmit (Tx) antenna and a single receive (Rx) antenna. In other words, MIMO technology allows a transmitting end and a receiving end to use multiple antennas so as to increase capacity or improve performance. If necessary, the MIMO technology may also be called multi-antenna technology.
A MIMO scheme includes a single codeword (SCW) method which simultaneously transmits N data streams (or N layers) using one channel encoding block and a multiple codeword (MCW) method which transmits N data streams using M (where M is equal to or less than N (where M≦N)) channel encoding blocks. Each channel encoding block generates independent codewords and each codeword is designed to independently detect errors. Each codeword may be mapped to one or more layers, and each layer may be mapped to a Tx antenna and then transmitted. In this case, specific information indicating which one of Tx antennas is mapped to each layer is determined by precoder information.
In a system for transmitting multiple codewords, a receiver needs to inform a transmitter of success/failure of detection (decoding) of each codeword. Thus, the receiver may transmit a hybrid automatic repeat request (HARQ) ACK/NACK signal for each codeword to the transmitter.
In case of uplink data transmission through a single antenna, single codeword (SCW) transmission can be supported. In addition, a synchronous HARQ scheme can be applied to single antenna uplink transmission, and an adaptive or non-adaptive HARQ scheme can be utilized according to whether a modulation and coding scheme (MCS) is changed during retransmission.